I guess not everone realizes that Mercedes-Benz are generally considered luxury vehicles, and command high prices. That being said, the parts will not be inexpensive--not for old cars, or new.
Arguably one of the least costly Mercedes Benz cars in recent year is the C class. A headlight assembly, or "door" as we come to call them, for a 2008 C300 lists for $1,250, and sells for about $800.
Even equivalent headlamp assemblies for pedestrian Toyota Camrys, Honda Accords, Ford Focus, etc. push $300 today.
So, an "equivalent" for a 38+ year old car costs about the same? I'm with Abe. We're darn lucky we can get them.